Two groups, each composed of five female rhesus monkeys in the first trimester of pregnancy, were studied to determine if the dietary ingestion of genistein, a plant hormone found in soy beans, had any effect on steroidogenesis in the maternal-fetal-placental unit. Monkeys in the experimental group were orally dosed with 8 mg of genistein per kg body weight each day, excluding Saturdays and Sundays, until day 155 of gestation. At that time the fetuses in both the experimental and control groups were delivered by c-section. Blood and urine samples were collected from all monkeys two times each week or every four weeks respectively during the pregnancy. Blood, urine, and fetal tissue samples were collected at the time of the c-section. Results showed no significant differences in the amount of weight gained by the mothers during the pregnancy, the weights of the fetuses at delivery, the weights of the placentas, or the progesterone levels in the mothers or fetuses. There were highly significant elevations of estradiol in the peripheral circulation of the mothers receiving genistein during the pregnancy and at the time of the c-sections. There were also significant elevations of estradiol in the blood from the ovarian veins and the ovarian-uterine veins in the genistein mothers at delivery and in the blood recovered from their fetuses. These data would suggest that the dietary genistein bound to estrogen receptors causing a competitive increase in the maternal and fetal production of estradiol in order to maintain the homeostasis required for pregnancy maintenance. The fetal increases would also indicate that the genistein crosses the placental barrier in a species of nonhuman primates that has a placenta similar to that found in humans. The possibility that high dietary intake of soy products containing genistein and other phytoestrogens could have an effect on fetal gonadal function in adulthood must be considered for human health reasons. Such evaluations were beyond the scope of the present study but are planned in a future grant application. This study supported by a grant from the Chancellor's Office, Tulane University Medical Center.